JPH06247330A - Rear wheel steering control device for use in torque sensitive four-wheel steering system - Google Patents

Abstract

PURPOSE:To enhance stability and convergence when a vehicle makes a turn by steering the rear wheels so that the response delay of a rear wheel steering actuator is canceled out. CONSTITUTION:In view of the fact that conventional rear wheel steering control becomes secondary delay control because of the response delay of an actuator (motor), a desired value deltar of rear wheel steering is given as the primary advance of the steering reaction of front wheels, to cancel out the response delay of the actuator so as to approximate rear wheel steering control laws to primary delay control that comprises ideal control laws as a whole (S5-S8.) In this case, the desired value deltar of rear wheel steering is computed from deltar=k1(v).Tr+k2(v).s.Tr wherein k1 (v) is a vehicle velocity sensitive torque proportional factor, k2 (v) is a vehicle velocity sensitive torque differential proportional factor, the k1 (v) and k2 (v) being the functions of vehicle velocity V, and s.Tr is the Lapras transform of the time differential of steering torque Tr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear wheel steering control device in a torque-sensitive four-wheel steering system (4WS) in which a rear wheel is steered by an actuator such as a motor according to a steering reaction force of a front wheel. Is.

[0002]

2. Description of the Related Art Recently, in a rear wheel steering control device of this type, as disclosed in, for example, Japanese Patent Laid-Open No. 3-148378, a steering torque sensor for detecting a steering reaction force is added to an electric power steering. A method has been proposed in which a rear wheel is steered by a motor according to the detection value of the steering torque sensor. Since this method does not use hydraulic equipment, it has the great advantage that the system can be made lighter, smaller and simpler. Further, at present, the electric power steering is mainly mounted on a light vehicle and some sports cars, and therefore, this method is considered to be the most mountable as a rear wheel steering method for these vehicle types.

[0003]

By the way, as described above, in the method of performing the rear wheel steering control by using the steering reaction force as the input data, the steering stability during the turning of the vehicle is shown by the following equation (1). As a control law of the rear wheel steering angle target value δr for realizing the above, the detected value of the steering torque sensor (hereinafter referred to as “steering torque”) Tr which is a steering reaction force Tr
It is considered that a control law with a first-order lag proportional to is ideal.

Δr = k1Tr / (1 + τs) (1) (k1: proportional constant, τ: first-order delay time constant, s: Laplace operator) Fig. 4 shows a simulation of the turning characteristics of a torque-sensitive 4WS when an ideal actuator (motor) that does not exist is used.
It is a graph of (b). This is the turning characteristic when the steering is turned at 30 ° in about 0.6 seconds at a speed of 100 km / h. At this time, the operation of turning the steering wheel is as shown in FIG.
It is represented by the curve of the front wheel steering angle.

A graph of FIG. 4A shows a simulation of the turning characteristic in the case of 2WS under the same conditions. The lateral G (lateral acceleration), the front wheel cornering force, the yaw angular velocity, and the center-of-gravity point sideslip angle shown in this graph are the physical quantities shown in FIG. That is, the center-of-gravity point sideslip angle β is the angle of deviation between the direction in which the vehicle is actually traveling (vector V) and the vehicle center axis, and the yaw angular velocity dψ /
dt is the angular velocity at which the vehicle rotates about the center of gravity, lateral G (ay) is the acceleration in the direction that makes an angle of 90 ° with the vehicle center axis applied to the center of gravity of the vehicle, and the front cornering force is applied to the left and right front wheels. It is the sum of forces Ffl and Ffr in the direction forming an angle of 90 ° from the vehicle center axis. This force is detected by the steering torque sensor as steering reaction force Tr.

Comparing FIGS. 4A and 4B,
In the case of 2WS of (a), after the yaw angular velocity has risen significantly, it has fallen significantly and overshoot has occurred, and after about 1 second, it is barely possible to settle to a value of about 0.9 ° / s. On the other hand, ideal torque-sensitive type 4
In the case of WS (b), the yaw angular velocity rises smoothly, the overshoot unlike 2WS does not occur, and it settles down to about 0.5 ° / s in about 0.4 seconds, so the driver looks like 2WS. You don't have to feel the uncomfortable overshoot caused by the yaw, and the vehicle operation is stable.

On the other hand, in terms of lateral G, 2WS is about 1
It is barely possible to settle down to about 0.42G in seconds, but in the case of an ideal torque-sensitive 4WS, it settles to about 0.26G in about 0.7 seconds, so the lateral G received by the driver is also small and the vehicle The operation is also stable. Also, the center-of-gravity point skid angle of 2WS is -1 ° in 0.6 seconds, but in the case of an ideal torque-sensitive 4WS, the turning direction is almost zero and the vehicle travels in line with the vehicle center axis. It is a characteristic. As described above, it can be seen that the ideal torque-sensitive 4WS shown in FIG. 4B has a turning characteristic in which the maneuverability and stability are significantly improved as compared with the 2WS.

However, in the above-mentioned conventional rear wheel steering control system, as shown in FIG. 6, the rear wheel is steered through the motor control unit 10 → motor 17 → rear wheel steering mechanism 18, so that the rear wheel has the aforementioned structure. There is always a response delay with respect to the rear wheel steering angle target value δr obtained by the equation (1). In particular, since the servo response characteristic of the motor 17 is greatly affected, the configuration shown in FIG. 6 actually results in a secondary delay response as shown in FIG. FIG. 3A is a graph simulating the turning characteristics under the same conditions as in FIG. 4 in consideration of this actuator response delay. In this case, the yaw angular velocity is suppressed to be smaller than that of 2WS, but it is found that the yaw angular velocity has a large overshoot as compared with FIG. Also,
The lateral G is also smaller than 2WS,
It seems to be overshooting, and it can be seen that this also lacks stability and convergence. Similarly, it can be seen that the sideslip angle of the center of gravity also appears as an overshoot.

In short, even if the rear wheel steering control law with the first-order lag shown in the equation (1) is applied to an actual vehicle, it does not actually look like FIG. 4 (b), and the actuator ( The response delay peculiar to the rear wheel steering mechanism including the motor) occurs, and this response delay causes a deviation from the ideal rear wheel steering state (see FIG. 4B) to occur, and )
As shown in (4), the stability and convergence during turning of the vehicle deteriorates, causing the driver to feel uncomfortable. Therefore, it is understood that the conventionally known rear-wheel steering rule with a first-order lag does not achieve the expected effect as long as this actuator (motor) is used.

The present invention has been made in consideration of such circumstances, and an object of the present invention is to take a response delay of an actuator for steering a rear wheel into consideration from the beginning and correct it. It is an object of the present invention to provide a rear wheel steering control device in a torque-sensitive 4WS capable of steering wheels and dramatically improving stability / convergence when turning a vehicle.

[0011]

In order to achieve the above object, a rear wheel steering control device in a torque-sensitive 4WS of the present invention detects an actuator for steering a rear wheel and a steering reaction force of a front wheel. A steering reaction force detecting means for controlling the actuator according to a detection value of the steering reaction force detecting means to steer the rear wheel, and to cancel the response delay of the actuator. It is configured to perform primary advance control.

[0012]

In the present invention, due to the response delay of the actuator,
Focusing on the point that the conventional rear wheel steering control is the second-order lag control, the rear wheel steering angle target is first advanced to the actuator by the steering reaction force of the front wheel (detection value of the steering reaction force detection means). In this case, the response delay of the actuator is canceled to bring the rear wheel steering control law closer to the first-order delay control which is an ideal control law [FIG. 4 (b)] for the entire control system. As a result, the poor stability / convergence of the conventional torque-sensitive 4WS as shown in FIG. 3A during turning of the vehicle is significantly improved as shown in FIG. Rear wheel steering control is performed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. The left and right front wheels 11L and 11R are, for example, racks &
It is steered by a pinion type electric power steering device 12. In this electric power steering device 12,
A steering shaft 13 and a steering wheel 14 are provided,
The steering shaft 13 is provided with a steering torque sensor 15 as a steering reaction force detecting means.

On the other hand, the left and right rear wheels 16L, 16R are steered by a motor 17 as an actuator via a rear wheel steering mechanism 18. The motor 17 is provided with a motor rotation angle sensor 19 such as a rotary encoder for detecting a rotation angle, and the rear wheel steering mechanism 18 is provided with a rear wheel steering angle sensor 20 for detecting a rear wheel steering angle. There is. The signals output from the rear wheel steering angle sensor 20, the motor rotation angle sensor 19, the steering torque sensor 15, and the vehicle speed sensor 21 are input to an electronic control unit (hereinafter referred to as “ECU”) 22.

The ROM (not shown) of the ECU 22 stores the 4WS control program shown in FIG. 2, and controls the motor 17 when the vehicle speed V is equal to or higher than a threshold value (for example, 40 km / h). 4WS control, which will be described later, is executed. Further, when the ECU 22 detects a system abnormality, the ECU 22 warns the driver with a warning lamp 23.

In this embodiment, the rear wheel steering angle target value δr is calculated by the following equation (2). δr = k1 (v) ・ Tr + k2 (v) ・ s ・ Tr (2) where k1 (v) is the vehicle speed sensitive torque proportional coefficient and k2 (v) is the vehicle speed sensitive torque differential proportional coefficient. k1 (v) and k2
(v) is a function of the vehicle speed V. Further, s · Tr is the Laplace transform of the time derivative of the steering torque Tr.

The purpose of using the equation (2) is as follows. That is, paying attention to the point that the conventional rear wheel steering control is the secondary delay control due to the response delay of the motor 17, and the rear wheel steering angle target value δr is set to the steering reaction force (steering torque Tr of the front wheels 11L, 11R). ), The response delay of the motor 17 is canceled, and the rear wheel steering control law is an ideal control law for the entire control system [Fig.
(B)], which is close to the first-order delay control.

In short, the transfer function of the second-order lag element 1 /
In order to convert {(s + ζ) · (s + η)} into the transfer function 1 / (s + ζ) of the first-order lag element, the transfer of the first-order lead element to the transfer function of the second-order lag element is expressed by the following equation (3). By utilizing the relationship that the transfer function of the first-order lag element is obtained as a whole by multiplying the function (s + η). 1 / {(s + ζ) · (s + η)} × (s + η) = 1 / (s + ζ) (3) In this case, in order to give the rear wheel steering angle target value δr in the form of a primary advance of the steering torque Tr. Is the above-mentioned (2)
The steering torque Tr and the time differential value s · Tr of the steering torque Tr may be linearly combined as shown in the equation.

Using the rear wheel steering control law (2) of the present embodiment derived based on such an idea, FIG. 3 (a) is used.
A graph simulating the turning characteristics under the same conditions as in FIG. By properly selecting the vehicle speed sensitive torque proportional coefficient k1 (v) and the vehicle speed sensitive torque differential proportional coefficient k2 (v) of the equation (2), the conventional convergence shown in FIG. It can be seen that this is improved and the state is approaching the ideal control state of FIG. For example, the yaw rate is
Some overshoot characteristics still remain, but Fig. 3
It is greatly improved compared to (a), and the overshoot characteristic is suppressed to a level where there is almost no problem. Further, the lateral G curve has almost the same overshoot and is almost in the same state as the lateral G curve of FIG. 4B. Further, the sideslip angle of the center of gravity is almost zero, and straightforward operating characteristics are realized. From these facts, it is understood that the rear wheel steering control law of this embodiment can realize the 4WS control close to the ideal torque-sensitive 4WS without the response delay of the motor 17.

Next, the contents of the 4WS control by the ECU 22 will be described with reference to the flowchart of FIG. First, after first performing an initial check in step S1, the vehicle speed signal output from the vehicle speed sensor 21 is read in step S2 to obtain the vehicle speed V. After that, it is determined whether the vehicle speed V is equal to or higher than a threshold value (for example, 40 km / h). If the vehicle speed V is lower than the threshold value, the following 4WS control is not executed and the process returns to step S2.

On the other hand, when the vehicle speed V becomes equal to or higher than the threshold value, the determination in step S3 becomes "YES", the process proceeds to step S4, and the 4WS control is executed as follows. First, in step S4, the steering torque Tr which is the output value of the steering torque sensor 15 is read, and in step S5
Then, the rear wheel steering angle target value δr is calculated by the above-mentioned equation (2).

Thereafter, in step S6, the output value of the motor rotation angle sensor 19 is read to determine whether or not the current position of the motor 17 matches the rear wheel steering angle target value δr (step S7). If they do not match (“NO”), the process proceeds to step S8, and the rear wheel steering angle target value δr
The positioning control of the motor 17 is performed according to
The position of is matched with the rear wheel steering angle target value δr. At this time, the position of the motor 17 may be detected by the motor rotation angle sensor 19, or the motor rotation angle sensor 19 and the rear wheel steering angle sensor 20 may be detected.
And may be used together to improve reliability. On the other hand, if “YES” in the step S7, that is, the motor 1
When the current position of 7 matches the rear wheel steering angle target value δr, it is not necessary to change the position of the motor 17, so the motor positioning control (step S8) is not executed and the process proceeds to step S9.

In step S9, a system abnormality check is performed. If the abnormality is normal, the process returns to step S2 to continue the above-described 4WS control, but if an abnormality is found, the process proceeds to step S10 and the motor 17 To stop the 4WS control, turn on the warning lamp 23 (step S11), and warn the driver.

According to this embodiment described above, the motor 1
Focusing on the point that the conventional rear wheel steering control is the secondary delay control due to the response delay of 7, the rear wheel steering angle target value δr is given in the form of the primary advance of the steering torque Tr. The response delay of the motor 17 is canceled out, and the rear wheel steering control law is an ideal control law for the entire control system [Fig.
(B)], which is close to the first-order lag control,
It is possible to dramatically improve the stability and convergence when the vehicle turns.

Although the motor 17 is used as the actuator in this embodiment, it is needless to say that the present invention can be similarly applied to a 4WS using an actuator having a second-order responsiveness.

[0026]

As is apparent from the above description, according to the present invention, the primary advance control is performed so as to cancel the response delay of the actuator. Therefore, the rear wheel steering control law is based on the control system. Ideal control law as a whole [Fig. 4
(B)], which is close to the first-order lag control,
It has an excellent effect that the stability and convergence when turning the vehicle can be dramatically improved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an entire 4WS system showing an embodiment of the present invention.

FIG. 2 is a flowchart showing a flow of 4WS control.

FIG. 3A is a graph showing a turning characteristic of a torque-sensitive 4WS in the case where a conventional actuator response delay is added;
(B) A graph showing the turning characteristics of the torque-sensitive 4WS using the rear wheel control law of the present embodiment that cancels the actuator response delay

FIG. 4 (a) is a graph showing a turning characteristic of 2WS,
(B) Graph showing turning characteristics of a torque-sensitive 4WS when an ideal actuator with no response delay is used

FIG. 5 is a diagram for explaining various physical quantities generated when the vehicle turns.

FIG. 6 is a block diagram of a control system

FIG. 7 is a diagram showing a conventional motor response characteristic.

[Explanation of symbols]

11L, 11R ... Front wheels, 12 ... Electric power steering device, 15 ... Steering torque sensor (steering reaction force detection means), 16L, 16R ... Rear wheels, 17 ... Motor (actuator), 18 ... Rear wheel steering mechanism, 19 ... Motor Rotation angle sensor, 20 ... Rear wheel steering angle sensor, 21 ... Vehicle speed sensor, 2
2 ... ECU.

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Claims (1)

[Claims] 1. An actuator for steering the rear wheels, and steering reaction force detection means for detecting the steering reaction force of the front wheels. The actuator is controlled according to the detection value of the steering reaction force detection means. A rear wheel steering control device in a torque-sensitive four-wheel steering system for steering the rear wheels is characterized in that primary advance control is performed so as to cancel the response delay of the actuator. Rear-wheel steering control device for torque-sensitive four-wheel steering system.